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    <title>Document</title>
    <script>
        //作用域链：内部函数访问外部，采用链式查找决定取那个值  就近原则
        var num = 10;

        function fn() { //外部函数
            var num = 20;

            function fun() { //内部函数
                console.log(num);
            }
            fun();
        }
        fn();
    </script>
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